A semi-conductor element may be a light emitting diode (LED) that is used for converting electrical energy into light energy. Two major parameters for light emitting diodes are the light intensity, which is the amount of optical power emitted per area, and the efficiency, which is the ratio of emitted optical power to the electrical power used by the device.
To produce blue or white light, gallium nitride light emitting diodes are frequently used. It is known that nanorods and microrods which have a core shell p-n-junction can be used to increase the active area of an LED. However, the efficiency of this kind of LEDs, presently is rather low.
The invention aims at increasing the efficiency and performance of gallium nitride semi-conductor elements, such as gallium nitride light emitting diodes, gallium nitride transistors, gallium nitride thyristors, etc.
The invention solves the problem by a method manufacturing of a semi-conductor element, and a semi-conductor element, as claimed in the appended claims.
Disclosed herein is a method of manufacturing of a semi-conductor element, comprising the following steps: (a) providing a substrate having a surface, the surface being partially coated with a coating and having at least one uncoated area, (b) growing a truncated pyramid of gallium nitride on the uncoated area, (c) growing at least one gallium nitride column on the truncated pyramid, (d) the step of growing the truncated pyramid is carried out such that the ratio of the area of the Ga-polar gallium nitride on the top surface of the truncated pyramid to the area of the N-polar region on the top surface of the truncated pyramid is decreasing with time, and (e) the growing of the truncated pyramid is stopped at a time when a top surface of the truncated pyramid is only N-polar.
Also disclosed is a semi-conductor element, in particular a LED, comprising (i) a substrate having a surface, (ii) a coating partially covering the surface, (iii) the surface having at least one uncoated area, and (iv) a truncated pyramid of gallium nitride on the uncoated area.
It is an advantage of the present invention that it increases the efficiency and performance of the semi-conductor element. With the prior art methods the middle part of a gallium nitride column grows on uncoated area of the substrate and also a part of the column grows on the coated substrate, which leads to a mixed-polarity growth. Mixed polarity means that some of the GaN-material grows in a N-polar c-direction and some other GaN-material grows in a Ga-polar c-direction. Therefore there are polarity inversion boundaries and dislocations which tend to scatter and absorb photons, thus reducing the efficiency of the light emitting diode as well as reducing transistor functions. With the new method, a gallium nitride column grown on a truncated pyramid leads to single polar gallium nitride column. The amount of polarity inversion boundaries and dislocations will be strongly reduced; therefore e.g. the light scattering and absorption in gallium nitride columns will also be reduced.
The method comprises the steps of (i) coating the substrate with the coating, (ii) transferring a pattern for the uncoated areas via lithography, for example photolithography or nanoimprint lithography, and (iii) etching, for example by inductively coupled plasma etching, the coating such that the surface of the substrate is partially coated with the coating and has at least one uncoated area. Techniques for coating, lithography and etching are generally known and not reproduced here.
The uncoated area may be hexagonal. The distance between two opposing sites of uncoated area may be between 100 nm to 50 μm. Generally, it is advantageous to have a small distance between two opposing sites, as this allows for increasing the density of uncoated areas and thus of gallium nitride columns, which increases the active surface of the semi-conductor element. However, the distance between two opposing sites must not be too small. If the p-doped GaN shell structure is grown on the n-doped GaN column (with or without a quantum well), a depletion region forms between the pn-junction. The diameter of the whole column should be larger than the width of the respective depletion region.
The pattern constant, i.e. the distance between geometrical centres of two uncoated areas may be between 200 nm to 50 μm.
In one embodiment, the truncated pyramid of gallium nitride on the uncoated area is grown in the crystallographic c-direction. That may be carried out in a 3×2″ FT Thomas Swan MOVPE system at 125 hPa using a carrier gas in the reactor of 50% N2 and 50% H2. To the carrier gas flow, NH3 and trimethygallium are added with a V/III ratio. The V/III ratio may be between 30 and 70, with 50 being the best value. The temperature may be T=960° C. This leads to a truncated pyramid having an angle between a ridge of the truncated pyramid and the substrate surface of about 58°. Depending on the growth time that may be between 100 and 260 seconds the height of the truncated pyramid may be between 100 nm and 400 nm.
The aspect ratio is the height of the gallium nitride column divided by the distance between two opposing sites of the gallium nitride columns. In certain alternatives, the aspect ratio is at least one.
It has been found that growing the truncated pyramid of gallium nitride and putting at least one gallium nitride column on top of the truncated pyramid can be carried out via metal organic vapor phase epitaxy (MOVPE).
The growth of the at least one gallium nitride column on the truncated pyramid may be carried out such that the gallium nitride column is only N-polar. Gallium nitride has a wurtzite type elementary cell. With respect to the crystallographic c-axis the gallium nitride elementary cell has an N-polar side and a Ga-polar side. Prior art methods lead to multi-polar gallium nitride crystals because when GaN is grown on this substrate, N-polar GaN will grow on the uncoated areas (or partly lateral over grow on the coated areas). Surrounding the N-polar GaN, Ga-polar GaN will grow on the coated area, due to the additional nucleation on the coating. The multi polar GaN columns have many polarity inversion boundaries between N-polar regions of the crystal and Ga-polar regions of the crystal. It has been found out that these polarity inversion boundaries cause absorption and scattering of light, thus reducing the efficiency.
In one alternative, growing of the truncated pyramid is carried out such that the part of the surface area of the GaN truncated pyramid which is covered by Ga-polar GaN crystals decreases to zero after a short growth time of truncated pyramid, the centre of the top surface of the GaN truncated pyramid being N-polar and the rim being Ga-polar. If we increase the growth time, the top surface area decreases with the increasing of the height of the truncated pyramid. The growth process is stopped when the top surface is covered just by N-polar GaN and the Ga-polar rim has disappeared. This can be achieved for example with the process parameters as described above. This method yields unipolar GaN column with a low dislocation density and high efficiency.
As an alternative, it is possible that the growth of the at least one gallium nitride column on the truncated pyramid is carried out such that the gallium nitride column has a N-polar core part and Ga-polar outer part surrounding the core part, the core part and the outer part contacting each other. This can be achieved by choosing a temperature between 1000° C. and 1040° C.
Alternatively or additionally, growing the at least one gallium nitride column on the truncated pyramid comprises n-doping the gallium nitride. For example, SiH4 may be used as an n-doping precursor.
According to one embodiment, the method comprises the following steps: (i) growing an InGaN/GaN quantum well shell structure and/or a p-doped shell structure surrounding the gallium nitride column at least partially, wherein the polarity of the quantum well and/or the p-doped gallium nitride are the same to the polarity of the sidewalls of the n-doped gallium nitride column, (ii) contacting the gallium nitride column electrically, and contacting the shell structure electrically. This leads to a method of manufacturing a light emitting diode.
It is possible to provide a substrate having a plurality of uncoated areas. In this case it is advantageous to grow one truncated pyramid of gallium nitride on a plurality of uncoated areas, in particular on each uncoated area. This allows for mass production of LEDs or other semi-conductor elements.
The method may comprise the step of removing the substrate at least locally and contacting the gallium nitride column electrically from both sides. For example, it is possible to grow a conducting layer such as an n-doped N-polar GaN layer below the pyramid for better contacts.
In various embodiments, the temperature is below or equal to 980° C. during growing of the truncated pyramid of gallium nitride. It is also preferred to carry out the growth process of the gallium nitride column at a temperature above 1050° C.
In various embodiments, the coating is a silicon oxide (SiOx) or silicon nitride (SiNx) coating. The substrate may be sapphire. It is possible to carry out the silicon dioxide coating process by plasma enhanced chemical vapour deposition. The coating thickness may be between 30 nm and 50 nm.
For a semi-conductor element according to the present invention, the height of the truncated pyramid is preferably larger than 100 nm. It also preferred that the height of the truncated pyramid is less than 400 nm. A high efficiency is obtained with monocrystalline and single polar gallium nitride columns.
The top cross section of a truncated pyramid parallel to the substrate surface may be smaller than the uncoated area it has been grown on. In this case, the gallium nitride column is usually unipolar. The gallium nitride column may be N-polar.
Preferred embodiments of the invention will now be described with reference to the accompanying drawing, in which.